Method for producing a schottky diode in silicon carbide

ABSTRACT

The invention concerns a method for making a vertical Schottky diode on a highly doped N-type silicon carbide substrate ( 1 ), comprising steps which consist in forming an N-type lightly doped epitaxial layer ( 2 ); etching out a peripheral trench at the active zone of the diode; forming a type P doped epitaxial layer; carrying out a planarization process so that a ring ( 6 ) of the P type epitaxial layer remains in the trench; forming an insulating layer ( 3 ) on the outer periphery of the component, said insulating layer partly covering said ring; and depositing a metal ( 4 ) capable of forming a Schottky barrier with the N type epitaxial layer.

[0001] The present invention relates to the forming of a Schottky diodein silicon carbide.

[0002] In the field of semiconductor components, the material which iscurrently mainly used is silicon. To withstand very high voltages,silicon carbide is a priori preferable since silicon carbide has abreakdown voltage per unit thickness approximately 10 times greater thansilicon.

[0003] However, in the present state of technologies, the processescurrently used to form silicon-based components cannot be transposed toform silicon carbide (SiC)-based components. In particular, it iscurrently not possible in practice to perform implantations anddiffusions of P-type dopants in N-type doped silicon carbide, notingthat the P-type dopant currently used for silicon carbide is aluminumand that the N-type dopant is nitrogen. Indeed, an anneal for diffusionof an implantation of a P-type dopant would require temperatures on theorder of 1700° C., which raises acute technological problems.

[0004] The elementary structure of a Schottky diode is illustrated inFIG. 1. This diode is formed from a heavily-doped N-type substrate 1 onwhich is formed an N-type epitaxial layer 2 properly doped to have thedesired Schottky threshold. On this epitaxial layer N is depositedsilicon oxide 3 defining a window in which the Schottky contact isdesired to be established by means of an adequate metallization 4. Therear surface of the component is coated with a metallization 5.

[0005] Such a structure has a very low breakdown voltage. Indeed, theequipotential surfaces tend to curve up to rise to the surface at theperiphery of the contact area, which results, especially in theequipotential surface curving areas, in very high field values, whichlimit the possible reverse breakdown voltage. To avoid thisdisadvantage, the structure shown in FIG. 2 in which a P-type peripheralring 6 is formed by implantation-diffusion at the periphery of theactive area of the Schottky diode is conventionally used forsilicon-based components. As a result, the equipotential surfaces mustpass in volume under the P regions and thus have a less markedcurvature. The diode breakdown voltage is considerably improved. As anexample with silicon having a 10¹⁶ at./cm³ doping level, the breakdownvoltage will be on the order of 10 V with no guard ring and on the orderof 50 V with a guard ring.

[0006] However, as previously indicated, the forming of such a P-typeguard ring is not simply implementable by implantation/diffusion in astructure formed on a silicon carbide substrate. In this case, thesimple structure illustrated in FIG. 1 is not desirable either, for thesame reasons as in the case of a silicon substrate.

[0007] Thus, the present invention aims at providing a method offormation of a Schottky diode having a relatively high breakdown voltagethat can be simply implemented when the semiconductor is siliconcarbide.

[0008] To achieve these objects, the present invention provides a methodfor manufacturing a vertical Schottky diode on a heavily-doped N-typesilicon carbide substrate, including the steps of forming alightly-doped N-type epitaxial layer; digging a trench peripheral to theactive diode area; forming a P-type doped epitaxial layer; planarizingso that a ring of the P-type epitaxial layer remains in the trench;forming on the external periphery of the component an insulating layerpartially covering said ring; and depositing a metal likely to form aSchottky barrier with the N-type epitaxial layer.

[0009] According to an embodiment of the present invention, this methodfurther includes the steps of forming at the same time as the peripheraltrench central grooves which are filled with portions of the P-typeepitaxial layer, to form a diode of Schottky-bipolar type.

[0010] According to an embodiment of the present invention, the N-typeepitaxial layer has a thickness on the order of a few μm and a dopinglevel on the order of 10¹⁶ atoms/cm³, and the P-type epitaxial layer hasa doping level on the order of 10¹⁶ atoms/cm³ and is formed in a trenchhaving a depth on the order of one μm.

[0011] The foregoing objects, features and advantages of the presentinvention, will be discussed in detail in the following non-limitingdescription of specific embodiments in connection with the accompanyingdrawings, in which:

[0012]FIG. 1 is a simplified cross-section view of an elementarySchottky diode;

[0013]FIG. 2 is a simplified cross-section view of a conventionalSchottky diode on a silicon substrate;

[0014]FIGS. 3A to 3C illustrate successive steps of formation of a dopedarea in a silicon carbide substrate;

[0015]FIG. 4 is a simplified cross-section view of a Schottky diode on asilicon carbide substrate; and

[0016]FIG. 5 shows an example of a Schottky-bipolar diode according tothe present invention.

[0017] As usual in the field of semiconductor representation, in thevarious drawings, the various layers are not drawn to scale, either intheir horizontal dimensions, or in their vertical dimensions.

[0018] As illustrated in FIGS. 3A to 3C, the present invention providesa succession of steps enabling formation of a doped area in a siliconcarbide substrate without requiring use of very high temperatures.

[0019] At the step of FIG. 3A, a trench has been formed in a siliconcarbide substrate 7. This trench has dimensions corresponding to thoseof the doped area which is desired to be formed and is formed by anyadapted photolithographic etch method. Substrate 7 is a massivesubstrate or an epitaxial layer formed on a support.

[0020] At the step of FIG. 3B, a doped epitaxial layer 8 of the desiredconductivity type, for example, of a type opposite to that of thesubstrate, has been formed. A P-type layer may for example be formed onan N-type substrate.

[0021] At the step of FIG. 3C, a planarization has been performed sothat there remains a portion 9 of the epitaxial layer in the trench. Thedesired result has then been obtained.

[0022] In a prior attempt to form a Schottky diode with a high breakdownvoltage based on a silicon carbide wafer, the applicant has provided inunpublished French patent application N^(o)99/16490 of the 24 Dec. 1999(B4379) the structure illustrated in FIG. 4.

[0023] The structure of FIG. 4 is formed from a heavily-doped N-typesilicon carbide wafer 11. A more lightly doped N-type thin epitaxiallayer 12 is formed on wafer 11. For a desired breakdown voltage on theorder of from 600 to 1000 V, this epitaxial layer has a thickness on theorder of from 3 to 6 μm. The Schottky contact is formed between thislayer 12 and a metallization 14, for example a platinum, titanium, ornickel silicide, or other. The rear surface of wafer 11 is coated with ametallization 5 corresponding to the diode cathode.

[0024] This structure is formed by performing the steps of:

[0025] forming a thin P-type doped silicon carbide epitaxial layer 15,the dopant being for example aluminum,

[0026] forming a peripheral trench 16 having substantially the depth ofthe sum of the thicknesses of epitaxial layers 12 and 15,

[0027] depositing a layer of a protection insulator 17, for examplesilicon oxide, and forming a central opening in which is formed Schottkymetal layer 14, which is thus in contact with N layer 12 and whichbiases P layer 15.

[0028] The distance between the periphery of the Schottky contact andthe trench is on the order of from 30 to 60 μm, for example, 40 μm.

[0029] The doping of P-type layer 15 is chosen so that, when a voltageclose to the maximum reverse voltage that the diode must withstand isapplied thereto, the equipotential surfaces, instead of all rising backup to the surface, extend at least partially to trench 16. For a diodeable to withstand from 800 to 1000 V, four equipotential surfacescorresponding to four values also distributed of the potential, forexample, values close to 200, 400, 600 and 800 V, have been shown. Itshould be noted that the equipotential surface substantiallycorresponding to 600 V reaches the trench.

[0030] The structure of FIG. 4 requires accurate adjustment of thedimensions and doping parameters, as well as a good quality of theinsulator covering the trench walls.

[0031] As a consequence, the present inventors have searched a way offorming a Schottky diode with a high breakdown voltage which, like thestructure of FIG. 4, can be formed on silicon carbide, but is easier toform.

[0032] To achieve this object, the present inventors have examined againthe structure conventionally formed on a silicon substrate shown in FIG.2 and, instead of trying to modify this structure, use, to obtain thisstructure, the method described in relation with FIGS. 3A to 3C.

[0033] Thus, to form the structure of FIG. 2 on a silicon carbidesubstrate, the present inventors start from a heavily-doped N⁺-typesilicon carbide substrate 1 on which a more lightly doped N-type siliconcarbide layer 2 is formed. The N dopant is for example nitrogen.

[0034] According to the present invention, in substrate 2, a peripheraltrench surrounding the active diode area is formed, after which a P-typedoped silicon carbide layer is deposited by epitaxy. The P dopant is forexample aluminum. After this deposition, a planarization is performed sothat there only remains ring-shaped P-type layer 6 present in thepreviously formed trench. This planarization is for example performed bychem-mech polishing. After this, an insulating layer 4 is deposited andetched, and metallization 4 is formed to obtain the structure shown inFIG. 2.

[0035] Thus, conversely to the state of the art on silicon in whichP-type ring 6 results from an implantation-diffusion, according to thepresent invention, P-type ring 6 results from an epitaxy. The fact thatthis P-type region results from an epitaxy instead of from animplantation-diffusion results in that this P-type region has ahomogeneous doping level while, when a structure results from animplantation-diffusion, it includes doping level unevennesses. Forexample, if the implantation is a surface implantation, the surfacedoping is heavier than the doping at the junction.

[0036] The present inventors have performed simulations on the obtainedstructure by using simulations methods known per se, and by using thesimulation program known as ISE-DESSIS, sold by ISE Company. Thesimulations have shown that, for a structure of the type of that in FIG.2, with an N-type epitaxial layer having a 12-μm thickness and a 8.1015at./cm³ doping level,

[0037] in the case of a structure with a P-type ring obtained byimplantation-diffusion of a 0.7-μm depth and of a 2.1017 at./cm³ dopinglevel, a 1205-volt reverse breakdown voltage is obtained;

[0038] in the case of a trench having a 1.5-μm depth filled with aP-type epitaxial layer having the same 2.1017 at./cm³ doping level, a1223-volt reverse breakdown voltage is obtained;

[0039] in the case of a trench having a 1.5-μm depth filled with aP-type epitaxial layer having a 5.1016 at./cm³ doping level, a 1415-voltreverse breakdown voltage is obtained.

[0040] A structure may for example be used in which the N-type epitaxiallayer has a thickness on the order of a few μm and a doping level on theorder of 10¹⁶ atoms/cm³, and the P-type epitaxial layer has a dopinglevel on the order of 10¹⁶ atoms/cm³ and is formed in a trench having adepth on the order of one μm.

[0041] Further, simulations show that, when a structure according to thepresent invention is urged to breakdown, the breakdown occurssubstantially in the middle of the P-type ring. Thus, this is a volumebreakdown and not a surface breakdown, and it is well known that themaximum possible theoretical breakdown voltage is then reached.

[0042] The structure according to the present invention has also beencompared to the structure illustrated in FIG. 4 and it has beenacknowledged that, there also, the breakdown voltage is better than insimilar conditions, that is, for a same doping level of the N-typeepitaxial layer in the case of the present invention with respect to thecase of FIG. 4.

[0043] In addition to this advantage of manufacturing ease and of betterbreakdown voltage of the structure of the present invention as comparedto the structure of prior art, it should be noted that another advantageof the present invention is that it is adapted to the forming of diodesof a specific type, known as Schottky/bipolar diodes. In such diodes,the metallization is in some places in contact with the N-type epitaxiallayer and forms therewith a Schottky contact, and in some places incontact with P-type regions and forms therewith an ohmic contact. Thedistance between N and P regions is calculated in a way known in the artto optimize the breakdown voltage and the diode rapidity.

[0044] Such a diode is shown in FIG. 5. It includes the same substrateand the same peripheral region 6 as the diode of FIG. 2. However, itfurther includes under contact 4, regularly spaced apart P-type dopedregions 20. These regions have for example concentric ring shapes or areparallel strips. According to the present invention, regions 20 resultfrom the forming of trenches at the same time as the forming of theperipheral trench. Thus, upon deposition of a P-type epitaxial layer,the P-type epitaxial layer fills the central trenches at the same timeas the peripheral trench. After chem-mech polishing or anotherplanarization step, the structure of FIG. 5 in which the peripheraltrench is filled with a portion of epitaxial layer 6 and the centraltrenches are filled with portions 20 of the same epitaxial layer issimply obtained.

[0045] Of course, the present invention is likely to have variousalterations, modifications, and improvements which will readily occur tothose skilled in the art, in particular as concerns the dimensions ofthe various layers, vertically as well as horizontally.

1. A method for manufacturing a vertical Schottky diode on aheavily-doped N-type silicon carbide substrate (1), including the stepsof: forming a lightly-doped N-type epitaxial layer (2); digging a trenchperipheral to the active diode area; forming a P-type doped epitaxiallayer; planarizing so that a ring (6) of the P-type epitaxial layerremains in the trench; forming on the external periphery of thecomponent an insulating layer (3) partially covering said ring; anddepositing a metal (4) likely to form a Schottky barrier with the N-typeepitaxial layer.
 2. The method of claim 1, characterized in that itfurther includes the steps of forming at the same time as the peripheraltrench central grooves which are filled with portions (20) of the P-typeepitaxial layer, to form a diode of Schottky-bipolar type.
 3. The methodof claim 1, characterized in that the N-type epitaxial layer has athickness on the order of a few μm and a doping level on the order of10¹⁶ atoms/cm³, and the P-type epitaxial layer has a doping level on theorder of 10¹⁶ atoms/cm³ and is formed in a trench having a depth on theorder of one μm.